The seats are okay and comfortable for an 8 hours drive. Our journey took a bit longer than expected. BUT it was freezing cold during the whole journey. Even though we've expected that and were prepared with jackets and long trousers, this was not enough. Bring a facial mask with you to not get infected.
IMPORTANT:
I recommend doing this as an experience to see how riding trains is in asia but i wouldnt recommend doing this regularily.
The trains in the first class are dirty, you can hear other people and vietnamese people sometimes tend to watch loud videos or sound music in the sleeping cabins. It is DEFINITELY not like first class in western countries, more like 3 third class travel for europeans.
The experience was great. The first class compartment is fine. There is food sold inside by a lovely lady. We didn't feel the 8 hours. Our train arrived 30min late at Nah trang but left on time.
The Saigon train station was a bit hard to find!